render: use provided character width

As a pretty large optimization, use the provided width when possible to
avoid measuring glyphs each render

Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>

src/cell.zig

@@ -6,6 +6,10 @@ pub const Cell = struct {
 
 pub const Character = struct {
     grapheme: []const u8 = " ",
+    /// width should only be provided when the application is sure the terminal
+    /// will meeasure the same width. This can be ensure by using the gwidth method
+    /// included in libvaxis. If width is 0, libvaxis will measure the glyph at
+    /// render time
     width: usize = 1,
 };
 

src/vaxis.zig

@@ -294,8 +294,12 @@ pub fn Vaxis(comptime T: type) type {
                 const cell = self.screen.buf[i];
                 defer {
                     // advance by the width of this char mod 1
+                    const w = blk: {
+                        if (cell.char.width != 0) break :blk cell.char.width;
+
                         const method: gwidth.Method = if (self.caps.unicode) .unicode else .wcwidth;
-                    const w = gwidth.gwidth(cell.char.grapheme, method) catch 1;
+                        break :blk gwidth.gwidth(cell.char.grapheme, method) catch 1;
+                    };
                     var j = i + 1;
                     while (j < i + w) : (j += 1) {
                         self.screen_last.buf[j].skipped = true;